Performance tweaks. optionally use CSS transitions. 

Firstly, thanks for this!

I've been trying to squeeze out performance gains and made this modification to use CSS transitions where available and if not fallback to normal positioning. In my tests it seems to help and reduces the number of layouts the browser needs to do (see images). Can be enabled/disabled through options.useCssTransform 
![before](https://f.cloud.github.com/assets/1655984/1695515/6a25e30e-5eb3-11e3-9225-5123a5d260de.png)
![after](https://f.cloud.github.com/assets/1655984/1695517/75515cae-5eb3-11e3-89d0-9f50485145d7.png)